The following code causes an assertion failure in PyErr_WriteUnraisable() (in Python/errors.c): class BadException(Exception): __module__ = None class BadClass: def __del__(self): raise BadException foo = BadClass() del foo this is because PyErr_WriteUnraisable() assumes that __module__ is a string, and passes it to _PyUnicode_EqualToASCIIId(), which asserts it received a string. what is the wanted behavior in such a case? should we ignore the bad __module__ and print '<unknown>' as the module name in the traceback?